Osteoporosis is a disease characterized by low bone mass and micro-architectural deterioration of bone tissue leading to enhanced bone fragility and a consequent increase in fracture risk. The disease causes bones to become fragile and brittle and affects both men and women. Osteoporotic bones increase the risk of fracture after minimal trauma. Globally there are an estimated 35 million women and 14 million men with osteoporosis or low bone mass. In the United States, one in four adults over the age of 50 is likely to suffer from an osteoporotic fracture. Osteoporosis and the resultant fractures represent a huge public health burden, in part because the disease strikes silently—by the time a patient is diagnosed with an osteoporotic fracture, the damage to bones has already been done.
Bone continually undergoes a process called remodeling. Bone loss occurs in osteoporosis because the normal process of remodeling, or bone turnover, removes more bone than it replaces. Bone remodeling involves two distinct stages: bone resorption (breakdown) and bone formation. Calcium is stored in bone. When it is needed in the body, bone cells called osteoclasts attach to the bone surface and break it down, leaving cavities in the bone. Bone forming cells called osteoblasts then fill the cavities with an organic matrix called osteoid. The osteoid then spontaneously mineralizes with calcium phosphate to reform the hard bone. Osteoblasts that remain embedded in the matrix are called osteocytes.
During aging and as a result of other conditions that may lead to increased risk of losing bone mass such as during treatment for prostate or breast cancer or as the result of malnutrition, the rate of bone turnover increases in both genders, and at the tissue level, osteoblastic bone formation is slower than osteoclastic bone resorption due to the decreased number and activity of individual osteoblastic cells (Marie and Kassem, 2011). Bone resorption takes less time than bone formation. Bone resorption at a particular bone site takes about two weeks; formation takes three months or more. As a result, there is a shortfall of bone at what are called remodeling spaces. Normally, this is of little consequence, but if the remodeling cycle is out of balance, bone turnover can result in major bone loss. High bone turnover is believed to increase fracture risk.
As a result, a need remains for methods for treating osteoporosis.